A light-emitting element containing an organic compound as a luminous body, which has features such as thinness, lightness, high-speed response, and DC drive at a low voltage, is expected to be applied to a next-generation flat panel display. In particular, a display device in which light-emitting elements are arranged in matrix is considered to have advantages in a wide viewing angle and excellent visibility over a conventional liquid crystal display device.
A light-emitting mechanism of a light-emitting element is as follows: a voltage is applied between a pair of electrodes where a layer containing an organic compound is interposed, electrons injected from a cathode and holes injected from an anode are recombined with each other at an emitting center of the organic compound layer to form molecular excitons, the molecular excitons release energy in returning to a ground state to emit light. Singlet excitation and triplet excitation are known as excited states, and light emission can probably be achieved through either of the excited states.
For a light-emitting device in which such light-emitting elements are arranged in matrix, a driving method such as passive matrix driving (simple matrix type) or an active matrix driving (active matrix type) can be used. However, when the pixel density is increased, the active matrix type where each pixel (or each dot) is provided with a switch is considered to be advantageous because it can be driven at a lower voltage.
The layer containing an organic compound has a structure typified by a stacked structure of a hole transporting layer, a light-emitting layer, and a electron transporting layer. EL materials for forming EL layers are roughly classified into low molecular (monomer) materials and high (polymer) molecular materials. An evaporation apparatus is used for film formation of the low molecular material.
A conventional evaporation apparatus includes a substrate is set on a substrate holder, and a crucible (or an evaporation boat) in which an EL material, that is, an evaporation material is sealed, a shutter to prevent the rising of a sublimating EL material, and a heater to heat the EL material in the crucible. Then, the EL material heated by the heater sublimates to form a film on the rotating substrate. At this time, a certain distance between the substrate and the crucible is needed in order to form a uniform film. For example, in a case of using a substrate having a size of 300 mm×360 mm, a distance of less than or equal to 1 m is needed. The larger the size of the substrate is, the longer distance is needed between the substrate and the crucible. The conventional evaporation apparatus requires a long distance between the substrate and the evaporation source in this manner; therefore, film formation speed is low and exhaust air in a film formation chamber takes a long time to lower throughput
Furthermore, in the conventional evaporation apparatus and a conventional evaporation method, in forming the EL layer by evaporation, most of the EL material that sublimates adheres to an inner wall of a film formation chamber included in the evaporation apparatus, the shutter, or a contamination shield (protection plate to prevent an evaporation material from adhering to the inner wall of the film formation chamber). Therefore, use efficiency of an expensive EL material is as extremely low as about 1% to increase manufacturing cost of the light-emitting device. In addition, regular maintenance such as cleaning to remove the EL materials that has adhered the inner wall of a film formation chamber and the contamination shield is needed; the maintenance involves temporary suspension of a partial manufacturing line even in a case of mass production.
Moreover, the conventional evaporation apparatus includes a film thickness monitor to obtain a uniform film. After an evaporation speed measured by the film thickness monitor becomes stable, evaporation to the substrate is begun with the shutter opened. Thus, there are loss of time taken until the evaporation speed stabilizes and loss of the evaporation material caused by the extra vaporization thereof.
Furthermore, in a case of using a large-area substrate, since the center of the substrate and the periphery thereof each have a different distance from the evaporation source, there is a problem in that film thickness tends to be uneven in these parts of the substrate. In addition, the evaporation apparatus that has a structure in which the substrate is rotated has a limit as an evaporation apparatus for the purpose of using a large-area substrate.
Moreover, in a case of forming and stacking of EL layers, since the conventional evaporation apparatus requires a certain distance between the substrate and the crucible, chambers each have a large capacity. Accordingly, after film formation of a first EL layer is completed, film formation of a second EL layer may cause the material of the first EL layer to enter the second EL layer.
Patent document 1 (Japanese Published Patent Application No. 2000-77182 bulletin Japanese Published Patent Application No. 2000-77182) discloses a technique in which an organic EL transfer substrate obtained by forming an organic compound having a light-emitting property on at least one surface of a heat-resistant film is heated by heat bars to transfer the organic compound having a light-emitting property to a transparent substrate having a transparent electrode.
Patent document 2 (Japanese Published Patent Application No. 2003-308974 bulletin Japanese Published Patent Application No. 2003-308974) discloses a technique in which a donor sheet including a chromogenic organic donor layer and a substrate that are in contact with each other are irradiated with radiant ray to form an organic material layer on the substrate.